The country’s biggest airline, IndiGo, on Monday placed a firm order for 500 Airbus A320 family aircraft, making this the largest purchase by an Indian carrier and eclipsing Air India’s recent order for 470 planes.The IndiGo order — worth almost $50 billion at list price, though IndiGo would have got a hefty discount thanks to the order size and the airline’s impeccable credentials — is also the largest-ever single aircraft purchase by any airline with Airbus.